Picosecond CARS measurements of nitrogen rotational/translational and vibrational temperature in a nonequilibrium Mach 5 flow

Picosecond Unstable-resonator Spatially Enhanced Detection Coherent Anti-Stokes Raman Scattering (USED-CARS) is used for the measurement of nitrogen Q-branch (Delta J = 0) spectra in the subsonic plenum and supersonic flow of a highly nonequilibrium Mach 5 wind tunnel. Spectra are processed to infer rotational/translational (T-rot) and first-level vibrational (T-vib) temperatures in the 200-370 torr plenum simultaneously. Operation of the nominally high reduced electric field (E/n(peak) similar to 500 Td), nsec pulsed discharge alone results in fairly significant vibrational loading, T-vib similar to 720 K/T-rot similar to 380 K; addition of an orthogonal low E/n (similar to 10 Td) DC sustainer discharge produces substantial vibrational loading, T-vib similar to 2,000 K/T-rot similar to 450 K. Effects of injection of CO2, NO, and H-2 downstream of the pulser-sustainer discharge are examined, which result in vibrational relaxation accompanied by simultaneous gas heating, T-vib similar to 800-1,000 K/T-rot similar to 600 K. CARSk measurements within very low-density flows in the Mach 5 expansion nozzle are also performed, with T-vib measured in both the supersonic free-stream and downstream of a bow shock created by a 5-mm-diameter cylindrical test object in the Mach 5 flow. Measurements within 300 mu m of the cylinder leading edge show that for pure N-2, or N-2 with 0.25 torr CO2 injection, no vibrational relaxation is observed behind the bow shock.